Here's the reason Bowser's voice has changed to much: not all of his voice actors are human beings.

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SOURCES
- https://www.gamecubicle.com/interview... ("Cinematic" Quote)
- https://www.facebook.com/139866783027... (Interview with Scott Burns)
-    • Jen Taylor & Kenny James - The Voices...   (Interview with Kenny James)
- http://www.gamespot.com/gamecube/acti... (Reviewer’s Quote)
- For sources relating to the SFX libraries, check the description of that video.

CREDITS
Mario Kart Wii Footage under creative commons via
   • Video  

MUSIC
All music via Epidemic Sound